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We present a detailed study of a molecular outflow feature in the nearbystarburst galaxy NGC 253 using ALMA. We find that this feature is clearlyassociated with the edge of NGC 253's prominent ionized outflow, has aprojected length of ~300 pc, with a width of ~50 pc and a velocity dispersionof ~40 km s^-1, consistent with an ejection from the disk about 1 Myr ago. Thekinematics of the molecular gas in this feature can be interpreted (albeit notuniquely) as accelerating at a rate of 1 km s^-1 pc^-1. In this scenario, thegas is approaching escape velocity at the last measured point. Strikingly,bright tracers of dense molecular gas (HCN, CN, HCO+, CS) are also detected inthe molecular outflow: We measure an HCN(1-0)/CO(1-0) line ratio of ~1/10 inthe outflow, similar to that in the central starburst region of NGC 253 andother starburst galaxies. By contrast, the HCN/CO line ratio in the NGC 253disk is significantly lower (~1/30), similar to other nearby galaxy disks. Thisstrongly suggests that the streamer gas originates from the starburst, and thatits physical state does not change significantly over timescales of ~1 Myrduring its entrainment in the outflow. Simple calculations indicate thatradiation pressure is not the main mechanism for driving the outflow. Thepresence of such dense material in molecular outflows needs to be accounted forin simulations of galactic outflows.

Dense Molecular Gas Tracers in the Outflow of the Starburst Galaxy NGC 253